There is a need for systems and methods of rejection of heat developed by lasers during operation. In particular, highly mobile, lightweight laser systems require energy efficient, compact heat transfer technology that is rugged.
For example, high-average power (HAP) solid-state lasers (SSL) are finding increasingly important utility in defense and contractors are now developing HAP-SSL for use in anti-missile defense, especially against artillery rockets and projectiles. Such HAP-SSL systems may be mounted on mobile platforms such as land vehicles, ships and aircraft. Mobile HAP-SSL may also address specialty industrial applications, including dismantlement of nuclear facilities, drilling of oil wells and road tunnels, and spacecraft orbit transfer.
Heat rejection in a laboratory SSL is typically accomplished by a conventional thermal management system (TMS) that uses two thermally coupled closed loops; a primary loop having a vapor compression-type heat pump rejecting heat to the environment (air or utility water) and a secondary loop with a recirculating liquid coolant. Such TMSs are large, heavy, expensive, and require large motive power to operate. These limitations make them unsuitable for use in military HAP-SSL applications, where rejection of large amount of heat is required promptly on demand.
Liquid nitrogen (LN2) may be also used as an expendable coolant since it can be readily evaporated and it is environmentally compatible. However, LN2 is storable for extended periods of time only if refrigeration is provided, which is impractical for many military systems. Ammonia and freons readily boil at ambient temperature and are storable. However, they are not environmentally acceptable. The liquid TMS noted above requires tanks with liquid coolant which are suitable for land vehicles and ships but, due to their size, weight and sensitivity to g-forces may be less suitable for aircraft platforms.
SSLs extract coherent light from an inverted population of excited neodymium, ytterbium, or other suitable lasant ions doped into crystals or glass. Population inversion is achieved by optically exciting lasant ions by absorption of optical radiation at wavelengths shorter than the laser wavelength. This process is commonly referred to as “pumping.” Depending on the excitation source and the lasant ions used, considerable portion (typically 10 to 50%) of the optical pump radiation is converted into heat and deposited into the SSL gain medium. For continuous operation, waste heat must be removed in real time by cooling selected surfaces of the laser medium. In addition, the source of optical pump radiation (typically semiconductor diodes) may also require cooling. Electro-optical efficiency of semiconductor diode for pumping SSLs is typically about 35-56%, where the balance is heat that must also be removed.
As a result, for every Joule of laser energy produced in a HAP-SSL, 2 to 6 Joules of heat must be removed from the laser. Thus, using a HAP-SSL generally includes the requirement to remove significant quantities of heat.
It is well known in the art that operating SSL materials and pump diodes at sub-ambient temperatures greatly improves device efficiency and improves thermo-mechanical/thermo-optical properties. In particular, at low temperature many important laser materials experience increased thermal conductivity, reduced coefficient of thermal expansion, and a reduced thermal dispersion coefficient (dn/dT), where n is the index of refraction and T is the temperature. Conventional refrigeration systems can be used to operate SSLs at sub-ambient temperature, but their size, weight and need for motive power make them unsuitable for certain applications, e.g., military HAP-SSL.
It is worthwhile to note that household refrigerators use a closed-cycle Joule-Thompson process. However, open-cycle Joule-Thompson cryogenic coolers are used in many commercial and military applications to reduce weight and power requirements. Innovations in the last decade include high efficiency heat exchangers fabricated by photolithography and the use of mixtures of gases rather than pure gases.
Photolithographically produced heat exchangers are characterized by their small size, low thermal mass, and low cost. Owing to their low thermal mass, these heat exchangers have demonstrated a capability for rapid cool-down from ambient to 80 degrees Kelvin in several seconds.
The most frequently used gases in Joule-Thompson cryogenic coolers are nitrogen or argon. However, recent experiments have demonstrated that mixing a small amount of high boiling point gases such as ethane, penthane or propane with nitrogen or argon may increase the Joule-Thompson refrigeration effect by a factor of 2 to 10. In addition, adding a small amount of Halon™ (CBrF3) renders the mixture non-flammable. For example, the integral iso-enthalpic refrigeration effect produced at 80 degrees K from a mixture of 83% nitrogen, 10% ethane and 7% propane vented from a tank with an initial pressure of 300 atmospheres to ambient back-pressure is about 2.4 kJ/mol of gas mixture, which translates to a refrigeration effect of about 71.4 kJ/kg. By increasing the tank pressure to 1,000 atmospheres (14,300 psi) the refrigeration effect is expected to increase significantly (possibly by as much as three-fold).
As a result, there is a need for rugged, energy efficient, light-weight, inexpensive HAP-SSL cooling systems because of the large quantities of heat that are released during lasing.